When a fuel cell assembly is to be used as a generator, it is necessary to supply hydrogen to a fuel cell assembly anode. A known method for acquiring hydrogen from gasoline, methanol, natural gas, or other hydrocarbon fuel by making use of reforming reaction is used to generate the hydrogen to be supplied to the anode.
There are various reforming reactions such as steam reforming reaction and partial oxidation reaction. As an example, the reforming reaction of isooctane (C8H18), which is an element of gasoline, is described below.C8H18+8H2O8CO+17H2  (1)C8H18+4O28CO+9H2  (2)
The reaction indicated by Equation (1) above is a steam reforming reaction. The reaction indicated by Equation (2) above is a partial oxidation reaction. The steam reforming reaction is an endothermic reaction. The partial oxidation reaction is an exothermic reaction. These reactions generally occur in a reactor vessel called a reformer. Either or both of these reforming reactions can be set to occur in a single reformer.
The operating temperature of a fuel cell assembly is approximately 80° C. for a proton exchange membrane fuel cell (PEM), which entails the lowest operating temperature, or as high as 1000° C. for a certain solid oxide fuel cell (SOFC). To generate power, therefore, it is necessary to warm up the fuel cell assembly to the operating temperature. A technology for supplying a cathode off-gas, which is discharged from a fuel cell assembly cathode, to the reformer for circulation purposes is disclosed by Japanese Patent Laid-Open No. 151599/2003. At system startup, the disclosed technology exercises control so that the amount of cathode off-gas supply to the reformer is about four times the air amount corresponding to the theoretical air-fuel ratio, and supplies high-temperature gas exhaust from the reformer to the anode for fuel cell assembly warm-up purposes.
However, when a cathode gas is supplied to the fuel cell assembly at the time of warm-up and the cathode off-gas exhaust from the fuel cell assembly is supplied to the reformer, the cathode gas unexpectedly cools the fuel cell assembly because the cathode gas is air having an ordinary temperature. Thus, the warm-up of the system including the fuel cell assembly takes a considerable amount of time. Consequently, it takes a long period of time before power is actually obtained from the fuel cell assembly.
The present invention has been made to solve the above problem. It is an object of the present invention to minimize the time required for warm-up when a fuel cell system is to be started up.